Before being able to be used in an industrial process, certain gases need to be stripped beforehand of the impurities that they contain.
Thus, atmospheric air, which generally contains approximately 250 ppm to 500 ppm of carbon dioxide (CO.sub.2) and a variable amount of water vapour and/or hydrocarbons, such as ethylene, must be stripped of its impurities prior to any cryogenic separation operations, especially a cryogenic distillation operation.
This is because, in the absence of such a pre-treatment of the air, the impurities CO.sub.2, and possibly H.sub.2 O and/or hydrocarbons, which are found in it would solidify at low temperature and would then contribute to the clogging-up of the heat exchangers and distillation columns, which would lead, on the one hand, to possible degradation of the equipment and, on the other hand, to incorrect separation of the various constituents of the air, such as nitrogen or oxygen.
Furthermore, the hydrocarbons likely to be present in atmospheric air may accumulate in the liquid oxygen in the cold box and there is then a risk of the plant deteriorating.
In fact, it is known that, in a cryogenic distillation column, impurities having a boiling point higher than that of oxygen and present in atmospheric air are liable to be concentrated in the liquid bath in the bottom of the column.
For obvious safety reasons, it is desirable to reduce the concentration of hydrocarbons in liquid oxygen to the lowest possible level.
Thus, cryogenic distillation plants are generally equipped with an air prepurification unit intended to stop most of the impurities present in atmospheric air.
To do this, several techniques and processes have already been proposed.
A first technique for removing CO.sub.2 and H.sub.2 O impurities contained in a gas stream, such as air, consists in refrigerating these impurities, that is to say in making the impurities solidify or crystallize at low temperature. However, this technique is little used as it is very punitive from the point of view of equipment and energy costs.
An alternative to this technique is to remove carbon dioxide, and possibly water vapour, contained in the gas stream to be treated by adsorbing these impurities on a suitable adsorbent, such as a zeolite or an activated alumina.
Zeolites and aluminas are in fact among the adsorbents most commonly used in adsorption-type gas separation processes.
Thus, document U.S. Pat. No. 3,885,927 describes the use of an X zeolite exchanged to at least 90% with barium cations, which zeolite has a CO.sub.2 adsorption capacity approximately 40% greater than an X zeolite containing only sodium cations.
Furthermore, document EP-A-284,850 describes a process for purifying a gas stream by adsorption on a faujasite-type zeolite, the Si/Al ratio is from 1 to 2.5, which faujasite is exchanged to at least 90% with divalent cations, such as strontium or barium cations.
Moreover, document U.S. Pat. No. 4,775,396 describes the preferential adsorption of carbon dioxide contained in a sweet gas, such as nitrogen, hydrogen and methane, using a PSA (Pressure Swing Adsorption) process with a fixed adsorption bed containing a faujasite-type zeolite exchanged to at least 20% with cations of the group formed from zinc, rare earths and ammonium and exchanged to at most 80% with alkali or alkaline-earth metal cations.
As regards document FR-A-2,335,258, this describes a process for purifying gas mixtures comprising nitrogen, hydrogen, argon and/or oxygen, and containing carbon monoxide, carbon dioxide and water impurities, by adsorption of the impurities on A- or X-type zeolites at a temperature of between -40 and +4.degree. C. This document describes A-type zeolites exchanged from 70 to 82% with calcium ions and the Si/Al ratio of which is at most 1, and exchanged or unexchanged X-type zeolites, the Si/Al ratio of which is between 1.15 and 1.5. Conventionally, X zeolites with a Si/Al ratio of less than 1.15 are called LSX (Low Silica X) zeolites or silica-depleted zeolites.
Likewise, document EP-A-0,718,024 describes the removal of CO.sub.2 contained in a gas stream by adsorbing the CO.sub.2 on an X zeolite, the Si/Al ratio of which is at most approximately 1.15, at a temperature of between -50.degree. C. and 80.degree. C approximately. This document furthermore describes the results obtained using X or LSX zeolites which are unexchanged or exchanged with lithium and calcium or rare earth cations.
Furthermore, document U.S. Pat. No. 1,357,053 describes the use of a mordenite-type zeolite exchanged with barium cations in order to remove nitrogen protoxide, the regeneration of the adsorbent taking place above 180.degree. C.
Moreover, document DD-A-6,225,345 recommends purifying the atmosphere for preserving foodstuffs by means of an adsorbent consisting of a mixture of active carbon and of an A zeolite exchanged with calcium cations.
However, the existing processes cannot be regarded as being completely satisfactory.
This is because some adsorbents, such as a standard 13X zeolite, normally used for removing CO.sub.2 and water vapour impurities contained in air, allow certain other compounds, such as ethylene, propane, methane, ethane and/or nitrogen protoxide, which are liable to be present in air, to be stopped only partially, or even not at all.
This is also reported in the document Linde Reports on Science and Technology, 36/1983, Dr J. Reyhing, "Removing hydrocarbons from the process of air separation plants using molecular-sieve adsorbers".
Similarly, the document MUST'96, Munich Meeting on Air Separation Technology, Oct. 10-11, 1996, Dr U. Wenning, "Nitrous oxide in air separation plants", emphasizes the ineffectiveness of 5A-type zeolites to stop nitrogen protoxide (N.sub.2 O) contained in air.
Thus, in general, when considering ethane, propane, nitrogen protoxide and ethylene impurities possibly present in air at variable contents, it may be stated that:
ethylene is an unsaturated hydrocarbon soluble in liquid oxygen up to a level of 30,000 ppm with a low solute-gas equilibrium coefficient. Its freezing point is -169.degree. C., whereas the temperature of liquid oxygen is -181.degree. C. at 1.2.times.10.sup.5 Pa. Therefore, when no air pre-treatment is carried out or this treatment is insufficient, ethylene may be incompletely stopped and in this case it may be found in solid form in the cryogenic plant; PA1 ethane and propane may be found in the liquid state at the temperature of liquid oxygen at 1.2.times.10.sup.5 Pa; and PA1 as for nitrogen protoxide (N.sub.2 O), this poses a different problem for cryogenic air separation units since this compound is found everywhere in the atmosphere at a level of 0.3 ppm and with an annual increase in content of the order of 0.3%. Unlike the abovementioned hydrocarbons, nitrogen protoxide is inert in liquid oxygen and represents no risk as it is. However, it is nevertheless liable almost always to get into the distillation columns of cryogenic separation units and it can then form solid deposits, as in the case of carbon dioxide, in the exchangers and distillation columns. It is therefore desirable to prevent such deposits, which may degrade the performance of the equipment. PA1 the at least one adsorbent comprising at least one zeolite is of X or LSX type having a Si/Al ratio of 1 to 1.25 approximately, preferably at least one LSX zeolite having a Si/Al ratio of the order of 1; PA1 the at least one adsorbent comprising a mixture of at least one A zeolite and of at least one zeolite is of X or LSX type; PA1 the X or LSX zeolite contains at least 30% of Ca.sup.2+ cations, preferably at least 50% of Ca.sup.2+ cations, and even more preferably from 60 to 95% of Ca.sup.2+ cations; PA1 it furthermore comprises at least one step of removing at least one impurity chosen from the group formed by water vapour, carbon dioxide, carbon monoxide, hydrogen and hydrocarbons, especially ethylene, propane and/or methane; PA1 the removal of at least some of the water vapour and carbon dioxide impurities is carried out over at least one bed of activated alumina particles; PA1 it is chosen from the group formed by TSA processes, preferably a TSA process carried out at a temperature of approximately -40.degree. C. to +80.degree. C.; PA1 it is carried out at a desorption pressure of 5.times.10.sup.5 to 10.sup.4 Pa, preferably between 1.4.times.10.sup.5 and 0.9.times.10.sup.5, preferably at a desorption pressure approximately equal to atmospheric pressure; PA1 it is carried out at an adsorption pressure of 10.sup.5 to 10.sup.7 Pa, preferably between 3.times.10.sup.5 and 6.times.10.sup.6 Pa; PA1 it comprises at least one step of regenerating at least one adsorbent, preferably at a regeneration temperature of 50 to 250.degree. C.; and PA1 the gas stream to be purified is air, preferably air subsequently separated by cryogenic distillation. PA1 the X or LSX zeolite furthermore contains cations chosen from alkaline-earth cations, preferably magnesium, strontium and/or barium cations; PA1 the adsorption is carried out in at least one adsorber and preferably in at least two adsorbers operating in parallel, that is to say one is in the production phase while the other is in the regeneration phase; PA1 it comprises at least one step of regenerating the adsorbent by flushing with residual nitrogen or another regeneration gas at a temperature of between 0.degree. C. and 300.degree. C., preferably at a temperature of between 50.degree. C. and 250.degree. C.
However, currently there is no really effective process for removing nitrogen protoxide (N.sub.2 O) impurities contained in a gas stream, particularly air, that can be used on an industrial scale.
This is because, although document EP-A-862,938 recommends the use of an unexchanged zeolite adsorbent, particularly of the 13X type, for removing NOx-type, especially N.sub.2 O, impurities, it turns out in fact that 13X zeolite is not effective, particularly for stopping N.sub.2 O, as will be demonstrated in the comparative tests carried out by the inventors of the present invention and presented below.
Moreover, another technique consists in purging or cold-adsorbing these impurities so as to prevent these impurities from reaching saturation level in the cryogenic separation unit.
However, this latter technique is very punitive from a costs standpoint, particularly energy costs, and therefore cannot be regarded as completely satisfactory.